Electrical and electronic circuitry is packaged for many reasons. Such packaging is important and essential to protect fragile circuit components from damage and to isolate the circuitry from its surrounding environments. The considerations to be resolved in packaging electrical and electronic circuitry include the protection of fragile circuit elements and connectors from breakage and other physical damage in handling and use, the prevention of unwanted transmission of electromagnetic radiation to the circuitry from the surrounding environment, and the containment of electromagnetic radiation generated in operation of the circuit. The latter two considerations generally require that the circuitry be surrounded by an enclosure of electrically conductive and magnetic material to isolate the circuitry electromagnetically from its environment.
These considerations have lead to over 60 years of inventive and developmental activity directed to circuitry shielding and packaging. A number of examples of such activity follow.
U.S. Pat. No. 1,641,395, for example, is directed to a composite shield for such radiation comprised of layers of a dielectric substrate, a thin metallic sheet, preferably half tin and half lead and a composite layer comprising preferably powdered borax and aluminum in an effort to provide rectification of radio energy and conduction to ground.
U.S. Pat. No. 2,321,587 discloses providing shielding on the glass envelope of a radio frequency electron tube with a composite coating of high electrical resistance.
U.S. Pat. No. 2,875,435 discloses a composite electromagnetic energy absorbing dielectric wall, comprising a metallic reflecting layer, a dielectric layer, a high-loss-producing layer, preferably a conductive plastic or rubber or a semiconductor, and a high refractive index dielectric tuning layer.
U.S. Pat. No. 2,992,425 discloses a composite non-directional electromagnetic radiation-absorbing material, comprising a metallic substrate and two layers of polymer with electrically conducting, elongated particles, with the conducting particles in one layer having their long axes lying at right angles to the long axes of the conducting particles in the other layer.
U.S. Pat. No. 3,638,148 discloses the addition to the lid of a container for a microstrip integrated circuit of an RF-absorbing, non-reflective material such as a three-layer, 3/8 inch (0.95 cm.) thick, polyurethane foam containing a resistive compound such a carbon particles, to provide approximately the effect of free space above the circuit within the container.
U.S. Pat. No. 4,218,578 discloses a radio frequency shielding for electronic circuits including a pair of spaced conductive enclosure portions carried by an encompassing dielectric body isolating electrically each conductive enclosure so that each enclosure portion may be grounded to a different dc ground without shorting out the different dc grounds.
U.S. Pat. No. 4,567,317 discloses an enclosure or housing for electrical or electronic circuitry comprising two interfitting box-like portions of dielectric material whose inwardly facing surfaces and interfitting surface portions are provided with a thin, metallized, electrically conductive coating. The circuit to be enclosed is placed within one of the box-like portions, and the other box-like portion is interfitted with the first box-like portion to enclose the circuit by continuous conductive interior walls whose outer surfaces are dielectric.
Notwithstanding the years of inventive and development effort, electrical circuits and microwave circuitry, as a practical matter, must often be contained in packages that are metallic or have conductive surfaces of low electrical resistivity.
Electrical and electronic engineers have long recognized that at ultra-high frequencies, the high-frequency energy within, for example, a container for a radio transmitter may be reflected within the transmitter and can create standing waves and a resonant cavity condition that can induce currents in the operating circuits that may be out of phase with desired operating currents and can modify intended circuit operation. U.S. Pat. No. 2,293,839, for example, discloses the addition of an energy absorbent material, such as fibers of conductive material like steel wool, to the reflecting surfaces of a grounded metal circuit container.
Packaging to shield or protect microwave circuitry presents a danger of substantial problems in the operation of the circuits, particularly where the circuits are large and operate at significant power levels. The packages themselves may act as resonant cavities and support resonant modes with high Q's that interfere with the desired operation of the packaged circuits.
Many circuits, especially monolithic microwave integrated circuits (MMIC's), must be placed in metal packages which are large enough to support resonant modes at their frequencies of operation. The frequencies of the resonant modes of a metal package decrease as the package dimensions increase, increasing the likelihood of interference with the enclosed circuit. If these resonant modes have a very high quality factor Q, as is usually the case, even a very loose coupling between the circuit and these modes can disturb circuit operation.
This problem has been addressed in at least one instance by decreasing certain package dimensions. U.S. Pat. No. 4,713,634 discloses a metallic container for a microwave circuit, including an interior cavity formed by metallic walls designed to increase the cutoff frequency of the waveguide propogation mode within the cavity above the operating frequency of the circuit. The metallic walls that form the interior cavity include sidewall portions, such as sidewall projections, that reduce the dimension of the cavity cross section that is parallel to the sidewalls with the circuit input and output, thereby decreasing the cutoff frequency wavelength and increasing the cutoff frequency of waveguide propogation mode above the operating frequency of the microwave circuit.
The undesirable interaction between the circuit and the resonant cavity modes of the package can also be reduced by dampening the resonant cavity modes. Conventional microwave absorbers composed of materials with bulk resistive properties may be placed in the package for this purpose, as, for example, in U.S. Pat. No. 3,638,148. Circuit reliability may be compromised, however, if microwave absorbers based on organic materials such as silicon rubber with a potential for outgassing are placed in the package with GaAs MMIC's. Furthermore, many microwave absorbers based on inorganic materials are difficult to machine to the small thicknesses required at microwave frequencies.
The inventor, in "Damping of the Resonant Modes of a Rectangular Metal Package", IEEE Trans. Microwave Theory and Techniques, Vol. MTT 37, No. 1, January 1989, has disclosed that the resonant modes of a rectangular metal package may be damped by fixing a dielectric substrate coated with a thin resistive film to one of its walls solve the reliability and machining problems associated with many conventional microwave absorbers. This is similar to the approach used in the Jaumann absorber disclosed in "Tables for the design of The Jaumann Microwave Absorber", Microwaves, Vol. 30, No. 9, pp. 219-222, September 1987, J. R. Nortier, C. A. Vander Neut, and D. E. Baker, in which resistive films supported by low dielectric substrates are placed at roughly quarter-wavelength intervals from a ground plane to suppress electromagnetic reflections; however, my technique differed, however, in that the substrates may have a high dielectric constant, may be much thinner, and are designed to suppress resonant modes rather than propagating waves.